There’s a recent letter to the journal Nature, which explores the age of single point mutations in the human genome. This data comes as part of a large DNA sequencing project, funded by the National Institutes of Health Heart, Lung and Blood Institute, which focuses on exons within the human genome.
Exons are kind of like the business end of the genome, meaning they make up the genetic code for proteins that play a major role in the development and everyday control of your body. That is not to say, however, that these are the only necessary or even the most important parts of the genome. As research progresses, biologists are learning that more and more of the DNA between these exons dictate where, when and how much of a protein will be made – regulation critical to life. But I digress.
The study, headed by the Akey group at the University of Washington, Seattle, examined the exomes (the part of the genome formed by exons) of 6,515 individuals from European and African decent. What’s the reason for looking at all of these exons? To compare the occurrence and position of small mutations across the genome. In doing so, the researchers were able to build a tree for the age, and therefore evolution, of mutations in our genome – specifically deleterious ones. And since deleterious mutations are sometimes correlated with disease, they were able to track, in a sense, the evolution of certain diseases.
In a nutshell, the study finds that most deleterious mutations are younger than 5,000 years old and that this can most likely be attributed to an explosion in the human population around that time. After all, the more genomes you make, the more you increase the chances for new mutations. The study also found that the surge of new mutations included among others, those responsible for premature ovarian failure, Alzheimer’s disease, coronary artery atherosclerosis and hereditary spastic paraplegia.
The study goes on to explore the acquisition of weakly deleterious mutations in the population that would become European, as they exited Africa between 50,000 and 150,000 years ago. This was caused by a bottleneck effect where the size of the migrating population decreased significantly after the separation from Africa. In the absence of variability in the genetic code of a population, non-lethal mutations can become somewhat ‘fixed’ in the genome making them commonplace among the individuals of that group, similar to the consequences of inbreeding.
What I find interesting about this study are its implications for the future. For instance, what about all of these recently acquired deleterious mutations? Why hasn’t evolution already weeded these out? And when will it? What does this study suggest about humans acquiring new mutations? And what about evolution; are humans actually still evolving?
According to the study in question, the deleterious mutations they found haven’t had significant time to be purged from the population. Let’s explore that for a minute. Evolution occurs when genetically healthy individuals pass their genetic code to a greater number of offspring than unhealthy individuals, thereby slowly eliminating deleterious genetic code. The amount of time it takes for this to happen is dependent on the type of deleterious code. For instance, something that severely affects the reproduction of young women will be eliminated very quickly whereas something that arises in postmenopausal women would have little effect on the next generation. We can assume, then, that most of the deleterious mutations uncovered in this study largely affect people after their main reproductive years; this is certainly the case with diseases such as coronary artery atherosclerosis and Alzheimer’s. One could even argue that premature ovarian failure wasn’t an issue 5,000 years ago when the average age of reproduction was significantly younger than today.
However, this does bring us to another of our questions; namely, will evolution ever weed out this deleterious genetic code? I would argue that the answer to that question is overwhelmingly…it depends. For the same reasons we just discussed, no one would predict an evolutionary elimination of Alzheimer’s, but due to an extension in childbearing years, premature ovarian failure may very well slowly disappear from the population. Of course, the elephant in the room is that this (meaning evolution) is, at this point, at odds with modern medicine. There are medical answers to weak fertility and premature death after all.
In a 2009 study by Stephen Stearns’ group at Yale University, the vital statistics of a population of women in Farmingham, Massachusetts were examined in relationship to the number of children each had. The results suggested that women that were slightly plump (but not obese) tended to have more children. You see being underweight with low blood pressure and low cholesterol decreases ovulation and therefore offspring. The prediction – woman in Farmingham will be about 1 inch shorter and 2.5 lbs heavier 10 generations from now.
My take on all of this? Humans are, without a doubt capable of evolving but I would go out on a limb and say what is possible is not probable.
I would argue that the data from the Stearns group study is irrelevant. What may have been a reproductive hurdle for women even 30 years ago is not necessarily a problem today and almost certainly less so in the future. In addition to this, current economical, cultural and environmental conditions are having an impact on number of offspring. For evolution to really gain traction in a population there must be medical conflicts and there can’t be moral or financial conflicts to the idea of unfettered procreation. In order for the predictions in Farmingham to hold up, plump, short women in the future are going to have to want more children, on average, than thin, tall women. Without that, the prediction falls apart. And we all know thin women that are exercise-inclined that have had healthy pregnancies. If being plump were a prerequisite for pregnancy, we’d have a new form of birth control to fall back on. No – I’m willing to bet that women in Farmingham in 2409 will look a lot like they did in 2009.
Advancements in medicine will allow more and more women to live and reproduce, despite their thinness and deleterious mutations.
This, coupled with further explosions in the world population will only compound the occurrence and long-term existence of deleterious mutations in the population; if not in undeveloped or underdeveloped countries, certainly in developed regions, which will become more commonplace. At this point, I think human evolution is going to go one of three ways: 1) stalled reproduction among the brightest, career-oriented people will cause a regression of society and general intelligence (the popularity of reality television is an omen and it’s further explained in this documentary). 2) Populations in poor, rural, undeveloped areas with little access to medical assistance will evolve into new species of humans, physically superior to homo sapiens. 3) And this one is certainly the most realistic; pollution, climate change, germ warfare or some other drastic, human-induced environmental catastrophe will eventually select for individuals that thrive in less oxygen, extreme temperatures or have relative immunity to trace levels of particular poisons.
In any case, unless the healthcare system collapses, we can expect a quick evolution of more deleterious mutations in our genomes and very little human evolution away from them. But then again, who needs evolution when you understand it enough to drive the process yourself? I’m sure Monsanto will have an evolutionary cure for a lot of our deleterious mutations soon enough.